A. Mechanisms for the transition to Castrate Resistant Prostate cancer Hypothesis: Following castration there is a decrease in prostate tumor volume and progression in animal models and human disease. However over time most tumors recur. The recurrence is associated with an increase in androgen regulated gene expression, increased androgen receptor expression, and ectopic production of androgens by prostate epithelial cells (1-3). Based on our preliminary data, we hypothesize that following castration there is an increase in the generation of splice variants of the androgen receptor that are missing all or part of the c-terminal ligand binding domain and are constitutively active. The specific variants we describe in the Work Accomplished have deletions of exons 5, 6, and 7 (AR del 5, 6, 7) or exons 5 and 6 (AR del 5, 6). These variant receptors are transcriptionally active in the absence of ligand and increase expression of genes necessary for androgen synthesis. When these events occur, generation of constitutively active AR and autocrine production of androgens, the tumor has successfully progressed to a castration-resistant state. In addition to the stimulation of androgen synthesis and AR transcription, the constitutively active AR suppresses IGF-IR gene expression. We hypothesize that inhibition of production or translocation of the AR and its splice variants will inhibit conversion to castrate resistant prostate cancer. Aim 1. Determine the timing of the increase in AR variants following castration and correlate the increase with the increase in wild-type AR, androgen synthesizing enzyme transcripts. Tumor measurements of androgens will be will be done in all specimens and correlated with splice variants, and synthesis enzyme expression. Aim 2. Determine the differences in gene expression when AR signaling is through the splice variant vs wt androgen ligated AR. Aim 3. Determine the role of splice variant ARs that increase following castration in the regulation of IGF-IR expression and the timing of effectiveness of histone deacetylase inhibitors or the inhibition of IGF-IR signaling on inhibition of prostate cancer growth as well as AR full length and variant expression. Aim 4. Determine the role of the AR-splice variant in progression of prostate cancer in an animal model. This aim will be started in year 1 and will require making a transgenic mouse expressing the AR 5, 6, 7 splice variant with a conditional probasin promoter. This mouse will be crossed with the TRAMP model of prostate cancer. The results of this study will define the role of constitutively active AR splice variants in the progression of prostate cancer following castration and effects of therapy targeting these variants.